I am confused by my GLIDE SPEED numbers
 

Aircraft is a -6A, with wheel pants and fairings installed.
Pitot and static system leak free, and seem to be reasonably accurate in level flight.

I am using the "FLIGHT TESTING HOMEBUILT AIRCRAFT" by Askue, and the "ILLUSTRATED GUIDE TO AERODYNAMICS" by Smith as the reference documentation for the performance testing of the aircraft.

I was expecting to see a fairly simple line graph plot of descent velocity, -fpm, (converted to glide ratio) vs airspeed. My plotted numbers do no look anything like I was expecting.

Flight conditions were as follows: max gross weight, Density Altitude = 4,980 ft. Flaps UP. Engine at idle. Slow to 80knots indicated and start the glide test.

As the airspeed stabilizes and the descent rate stabilizes, record the numbers and slow to the next IAS. Repeat...

Here are the numbers:
IAS(knots) Descent Rate (fpm)
85 650
80 700
75 700
70 650
65 700
60 650
58 full stall

When the numbers are plotted out on a graph, I end up with a sine wave sort of plot, with several minimum descent rates at different IAS numbers.

Does this make any sense?? I think I understand how to get Minimum Sink Rate and Max Distance Glide from a smooth plot curve, but not from the data /plots I have captured. Anyone have any words of wisdom here??

I have also performed this flight test at max gross weight, FULL FLAPS, same density altitude. Although the numbers are different, they follow the same general trend of plotting a sine wave of -fpm vs IAS.

Oh, as a comment, I have two different ASI installed. One is a standard Van's round steam gauge, the other is a Dynon D10A. All of the numbers presented are from the steam gauge. It is the one I use because it is easier for me to read. The Dynon unit presents more accurate info at the lower end of the dial, but it to follows the sine wave numbers when plotted out...just different numbers. The actual Stall point according to the D10A is 53 knots w/ flaps up.

58kt vs 53 kt at stall is a big difference! I'd go with they Dynon personally, I'd assume they're more accurate. Does it display altitudes also?

What I understand about glide performance testing, they do a sawtooth-like profile. What I'd do is set your alitmeter to 29.92"HG to get pressure altitude. Climb to a few thousand feet above ground and record the OAT. Start the descent and once your stabilized, time the difference between two altitudes. Say going from 6000' to 5000' for example. That would give you a more accurate vertical speed. Then climb up and do it again at a different IAS. On the ground you can correct for density and temperature variations.

The problem you're having is that the VSI isn't very accurate and 50fpm is too coarse of a measurement to be able to get data from.

The error may be in the sink rate numbers.

Were they taken by a reading a standard VSI?

Timing a descent and reading altitude over xx seconds may be more accurate, or better still, a glider variometer.

And you should be able to pull the altitudes off of the Dynon datalogger. Use the altitudes to obtain your descent rate. Use a running average of descent rates rather than point values. I would trust the Dynon data much more than the steam gauges, and the Dynon data give you many more points which will result in a better average. This should help give you a better curve.

greg

The Dynon unit displays DA in real time. It can completely replace all of the traditional steam gauges and then some. I just find the steam gauge easier to read. In the referenced documentation in my first post, Glide Performance
is not the same a Climb Performance as engine power / cooling issues are not a factor. Then, by calculating the Glide Ratio using the sink rate / speed x 1.689, I still get a funkylooking graph.

When recording the data, I copy both steam gauge and Dynon as a common line item, and then correlate the data on the ground. If I use just the data from Dynon, I get the same basic sine wave shape...the values are shifted by about 8 knots at the low end of the speed range to about 3 knots at the high end, where "low = 55knots & high = 155knots".

The vertical speed indicator is easy to read, the Dynon bar graph not so much so. I may try the stopwatch task to see if it correlates the altimeter / VSI etc. The Dynon altimeter and the steam gauge altimeter are in lock step, just different by about 40 ft.

I have "calibrated" the Dynon vs Steam gauge using a manometer, and according to the manometer, Dynon is very accurate, the steam gauge not so much. Regardless of which instrument is used, the vertical speed profile does not graph into a "smooth" line plot.

One other thing is you are measuring the minimum sink rate, which is not necessarily the maximum glide range speed.

The minimum sink rate glide is very much similar to the best rate of climb speed, they both depend on the power required. In this case, power is the speed * drag force. Minimum power yields the least sink rate. Maximum power yields the highest climb rate.

Maximum glide ratio or "flattest glide speed" is approximately 1.31* Vmin sink

Do not use the VSI for climb or descent performance testing. As mentioned pick a target altitude say 3000 ft PA, climb to 4500 ft Pa and establish at stabilized descent. Start timing at 4000ft PA and stop timing at 2000 ft ( a 2000 ft block) . Maintain airspeed +- 2 kits ball centered throughout. Note vsi at 3000 ft (your target altitude). Repeat for each airspeed through the range of speeds of interest. You need to be quick to minimize the error due to change in weight (fuel burn) . Post flight simply divide your alt block by the time . This is the average Rate Of Descent at 3000ft PA. Compare this to the VSI and you'll often note a significant diff. To maximize effeciency, stay trimmed at the airspeed add full power and climb back through the same alt block recording the same values to obtain climb rate data. This is in fact called Sawtooth climb and descent testing for obvious reasons. If you have a very accurate drag polar you can negate the effects of fuel burn mathematically but isn't necessary because the change in value isn't significant enough to worry about if you proceed quickly from one airspeed to another.

Static errors
 

The most likely cause for your 'confusion' is static error, which is causing the inconsistent readings (even if your instruments are perfect). Static readings are notoriously incaccurate at low speeds. The usual result is artificially low airspeed readings - you are actually going faster than the ASI reports, so at least the inaccuracy is on the 'safe' side. The effect on the VSI and Altimeter is moot at a constant airspeed since you are measuring deltas, but turbulence or airspeed variations will cause data variations in them as well of course.

Not much you can do about it unless you want to go to some trouble.

A 'few' years ago I took a college flight test course (I'm an AE) using a Cessna 206 equipped with a long (~100+ ft. maybe 200, it has been a while) plastic tube that was reeled out in flight. A funnel was attached to the end of the tube as a 'drag chute' to keep it stable, and about 10 ft forward of the funnel was a hole to get a static reading. This was a cheap and easy way to get a static reading with minimal effect from the airplane. The plane had a whole set if PS instruments in back for taking readings and comparing them to the aircraft instruments (this was before PC's much less EFIS's :eek: ).
You'd have to plumb the trailing line into your static system somehow and do your testing with that static source.

I've never gone to such efforts on my plane. :rolleyes:

The problem that I had with V-Speed testing is that it is very difficult to do with other air traffic around or with any kind of turbulence.

Corrective inputs from the flight controls will increase drag and invalidate your measurements.

Best to go up very early in the morning, with no traffic and still air. Always trim to the test speed (no pitch input on the stick).

V

The brainless way, if you happen to have a Garmin x96 GPS:

Use the menu options on the flight parameter page to display "glide ratio".

Vary your airspeed until the displayed glide ratio is maximized

Done deal.

erich